No proper minerals of the rare element rhenium are yet known. Instead, it occurs as a minor element in molybdenite or as traces in columbite, gadolinite and some manganese, platinum and uranium ores. The largest amount of Re is isolated by processing the roasting gases from molybdenum and copper production, although recycling of Re from secondary raw materials containing rhenium, for example, spent catalysts, is also possible.
As a rule, solutions containing rhenium which are contaminated with other elements are ultimately obtained, and are subsequently enriched by means of ion exchange, extraction, precipitation crystallisation or electrolysis and processed. As ion exchangers, U.S. Pat. Nos. 2,876,065 and 3,672,874 recommend the use of strongly basic anion exchangers, as does U.S. Pat. No. 3,733,388. The aforementioned U.S. Pat. No. 2,876,065 teaches bringing the Re solution, still containing Mo and other impurities (such as As and Se), into contact with a strongly basic anion exchanger after a pre-purification stage, in which case Mo as well as, for example, As and Se are also taken up besides Re. The molybdenum, together with the impurities, are therefore first alkalinically released, and only after a washing step is the rhenium remaining on the exchanger eluted by means of dilute solutions of strong mineral acids (HCl, H2SO4, HNO3 and HClO4), perchloric acid being presented as the most effective of those mentioned. Disadvantages in this case, however, are that the eluted Re product, perrhenic acid, is obtained as a mixture with perchloric acid and further elaborate separating processes therefor are necessary. The presence of perchloric acid places stringent requirements on safety standards and on material properties of the equipment which is used; moreover the perchlorate thereof becomes bound almost irreversibly to the anion exchanger during the elution step. This means that the exchanger cannot be reused with full capacity (substitution after a few sorption-elution cycles). U.S. Pat. No. 2,945,743 and SU patent 163 359 also propose the use of strong mineral acids as eluents.
SU patent 193 724 describes a hydrazine solution (8% strength) as eluent in order to improve the ability to regenerate the anionic exchanger, although the desorbed perrhenate needs to undergo an additional extractive purification step (treatment with tributyl phosphate). At the same time, the technical handling of hydrazine is found to be relatively demanding.
Other patents, such as U.S. Pat. No. 3,558,268 and DE-A 1 808 707, use an aqueous thiocyanate solution as eluent, the use of thiocyanate itself is problematic in environmental terms. A particular disadvantage is that only thiocyanates with significant water solubility can be used, and their solubility in water must be higher than that of the corresponding perrhenates (when both salts are present in solution).
A favourable perrhenate-electrolyte ratio is achieved by DD 260 227 A1 through the use, as eluent, of mixtures of a nitric or hydrochloric acid electrolyte and an organic solvent which is miscible with aqueous solutions, such as those from the group of ketones, or alcohols. The use of organic reagents with the associated problem of handling/hazard potential/disposal/etc. constitutes as much of a disadvantage in this variant as the low yield of rhenium does.
A number of prior publications concentrate on Re elution with the use of hydrohalic acids, preferably aqueous hydrochloric acids. Illustratively, DE 28 36 641 A1, DE 28 36 632 A1 and DD 155 825 use preferably 4 to 8 M hydrochloric acid at elevated temperatures (50-100° C.) as the basic operation for Re elution from strongly basic anion exchangers. They, however, show comparatively high residual Re contents on the exchanger materials, and hence low Re yields, and also—owing to the elevated operating temperatures—short exchanger lives. DD 155 825 describes pure hydrochloric acid elution, which is associated with the aforementioned disadvantages, with circulation of the hydrochloric acid and, therefore, combined perrhenate crystallisation in the bottom fraction.
It is an object of the present invention to provide a process which avoids the disadvantages of the known processes. In particular, it is an object of the present invention to extend the life of ion exchangers, that is to say the cycle number for regeneration of ion exchangers. It is a further object of the present invention to increase the elution level and therefore the loading/discharge capacity of ion exchangers.